A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning device (e.g., a mask) may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising part of, one or several dies) on a substrate (e.g. a silicon wafer) that has a layer of radiation-sensitive material (resist). Instead of a mask, the patterning device may comprise a patterning array that comprises an array of individually controllable elements. An advantage of a system using a patterning array compared to a mask-based system is that the pattern can be changed more quickly and for less cost.
In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion in one go, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction.
A lithographic apparatus typically comprises an illuminator configured to provide a conditioned illumination beam of radiation. In order to provide a shaped illumination beam, one or more masks may be provided within the illuminator in order to block off portions of the illumination beam, thus changing the pupil shape of the illumination beam. Alternatively or additionally, an array of individually controllable elements, such as a programmable mirror array, arranged to selectively reflect portions of the illumination beam may be provided in order to create a shaped illumination beam that can be controlled, and thus readily changed from one cross sectional pattern to another. However, the illumination beam typically operates at a relatively high intensity, such that, for example, the array of individually controllable elements may become heated. Such heating may, for example, affect the accuracy of the beam path along which the illumination beam is selectively reflected, thus causing the shaped illumination beam to deviate from the expected shape.